Spinal fusion implants and tools for insertion and revision

ABSTRACT

An interbody fusion device in one embodiment includes a tapered body defining a hollow interior or chamber for receiving bone graft or bone substitute material. The body defines exterior threads which are interrupted over portions of the outer surface of the device. The fusion device includes truncated side walls so that on end view the body takes on a cylindrical form. In another embodiment, the tapered body is solid and formed of a porous biocompatible material having sufficient structural integrity to maintain the intradiscal space and normal curvature. The material is preferably a porous tantalum composite having fully interconnected pores to facilitate complete bone tissue ingrowth into the implant. In further embodiments, the fusion devices are provided with osteogenic material to facilitate bone ingrowth. A cap is also provided to block the opening of hollow fusion devices. The cap includes an occlusion body and an elongated anchor. In some embodiments the anchor includes a lip which is engageable to openings in the body wall. Tools are also provided for manipulating caps for interbody fusion devices. In one embodiment the tool includes a pair of prongs each having facing engagement surfaces for engaging the fusion device, and a shaft slidably disposed between the prongs. The shaft has a first end defining a cap-engaging tip for engaging a tool hole in the cap. In one embodiment the cap engaging tip defines threads. In another embodiment the prongs include a pair of releasing members on each of the facing engagement surfaces. The releasing members have a height and a width for being insertable into apertures in a body wall in the fusion device to disengage the elongate anchors from the apertures.

The present application is a continuation of Ser. No. ______ (attorneydocket # 4002-740), filed on Feb. 11, 1997, which is acontinuation-in-part of co-pending application Ser. No. 08/603,674,filed on Feb. 19, 1996 which is a continuation-in-part of co-pendingapplication Ser. No. 08/413,353, filed on Mar. 30, 1995 which is acontinuation-in-part of co-pending application Ser. No. 08/411,017,filed on Mar. 27, 1995.

BACKGROUND OF THE INVENTION

The present invention relates to an artificial implant to be placed intothe intervertebral space left after the removal of a damaged spinaldisc. Specifically, the invention concerns an implant that facilitatesarthrodesis or fusion between adjacent vertebrae while also maintainingor restoring the normal spinal anatomy at the particular vertebrallevel.

The number of spinal surgeries to correct the causes of low back painhas steadily increased over the last several years. Most often, low backpain originates from damage or defects in the spinal disc betweenadjacent vertebrae. The disc can be herniated or can be suffering from avariety of degenerative conditions, so that in either case theanatomical function of the spinal disc is disrupted. The most prevalentsurgical treatment for these types of conditions has been to fuse thetwo vertebrae surrounding the affected disc. In most cases, the entiredisc will be removed, except for the annulus, by way of a discectomyprocedure. Since the damaged disc material has been removed, somethingmust be positioned within the intradiscal space, otherwise the space maycollapse resulting in damage to the nerves extending along the spinalcolumn.

In order to prevent this disc space collapse and to stabilize the spine,the intradiscal space is filled with bone or a bone substitute in orderto fuse the two adjacent vertebrae together. In early techniques, bonematerial was simply disposed between the adjacent vertebrae, typicallyat the posterior aspect of the vertebrae, and the spinal column wasstabilized by way of a plate or a rod spanning the affected vertebrae.With this technique once fusion occurred the hardware used to maintainthe stability of the segment became superfluous. Moreover, the surgicalprocedures necessary to implant a rod or plate to stabilize the levelduring fusion were frequently lengthy and involved.

It was therefore determined that a more optimum solution to thestabilization of an excised disc space is to fuse the vertebrae betweentheir respective end plates, most optimally without the need foranterior or posterior plating. There have been an extensive number ofattempts to develop an acceptable intradiscal implant that could be usedto replace a damaged disc and yet maintain the stability of the discinterspace between the adjacent vertebrae, at least until completearthrodesis is achieved. These “interbody fusion devices” have takenmany forms. For example, one of the more prevalent designs takes theform of a cylindrical implant. These types of implants are representedby the patents to Bagby, U.S. Pat. No. 4,501,269; Brantigan, U.S. Pat.No. 4,878,915; Ray, U.S. Pat. Nos. 4,961,740 and 5,055,104; andMichelson, U.S. Pat. No. 5,015,247. In these cylindrical implants, theexterior portion of the cylinder can be threaded to facilitate insertionof the interbody fusion device, as represented by the Ray, Brantigan andMichelson patents. In the alternative, some of the fusion implants aredesigned to be pounded into the intradiscal space and the vertebral endplates. These types of devices are represented by the patents toBrantigan, U.S. Pat. Nos. 4,743,256; 4,834,757 and 5,192,327.

In each of the above listed patents, the transverse cross section of theimplant is constant throughout its length and is typically in the formof a right circular cylinder. Other implants have been developed forinterbody fusion that do not have a constant cross section. Forinstance, the patent to McKenna, U.S. Pat. No. 4,714,469 shows ahemispherical implant with elongated protuberances that project into thevertebral end plate. The patent to Kuntz, U.S. Pat. No. 4,714,469, showsa bullet shaped prosthesis configured to optimize a friction fit betweenthe prosthesis and the adjacent vertebral bodies. Finally, the implantof Bagby, U.S. Pat. No. 4,936,848 is in the form of a sphere which ispreferably positioned between the centrums of the adjacent vertebrae.

Interbody fusion devices can be generally divided into two basiccategories, namely solid implants and implants that are designed topermit bone ingrowth. Solid implants are represented by U.S. Pat. Nos.4,878,915; 4,743,256; 4,349,921 and 4,714,469. The remaining patentsdiscussed above include some aspect that permits bone to grow across theimplant. It has been found that devices that promote natural boneingrowth achieve a more rapid and stable arthrodesis. The devicedepicted in the Michelson patent is representative of this type ofhollow implant which is typically filled with autologous bone prior toinsertion into the intradiscal space. This implant includes a pluralityof circular apertures which communicate with the hollow interior of theimplant, thereby providing a path for tissue growth between thevertebral end plates and the bone or bone substitute within the implant.In preparing the intradiscal space, the adjacent end plates arepreferably reduced to bleeding bone to facilitate this tissue ingrowth.During fusion, the metal structure provided by the Michelson implanthelps maintain the patency and stability of the motion segment to befused. In addition, once arthrodesis occurs, the implant itself servesas a sort of anchor or scaffold for the solid bony mass.

A number of difficulties still remain with the many interbody fusiondevices currently available. While it is recognized that hollow implantsthat permit bone ingrowth into bone or bone substitute within theimplant are an optimum technique for achieving fusion, most of the priorart devices have difficulty in achieving this fusion, at least withoutthe aid of some additional stabilizing device, such as a rod or plate.Moreover, some of these devices are not structurally strong enough tosupport the heavy loads and bending moments applied at the mostfrequently fused vertebral levels, namely those in the lower lumbarspine.

There has been a need for providing a hollow interbody fusion devicethat optimizes the bone ingrowth capabilities but is still strong enoughto support the spine segment until arthrodesis occurs. It has been foundby the present inventors that openings for bone ingrowth play animportant role in avoiding stress shielding of the autologous boneimpacted within the implant. In other words, if the ingrowth openingsare improperly sized or configured, the autologous bone will not endurethe loading that is typically found to be necessary to ensure rapid andcomplete fusion. In this instance, the bone impacted within the implantmay resorb or evolve into simply fibrous tissues rather than a bonyfusion mass, which leads to a generally unstable construction. On theother hand, the bone ingrowth openings must not be so extensive that thecage provides insufficient support area to avoid subsidence into theadjacent vertebrae.

The use of bone graft materials in past metal cage fusion devices haspresented several disadvantages. Autograft is undesirable becauseexisting structures may not yield a sufficient quantity of graftmaterial. The additional surgery to extract the autograft also increasesthe risk of infection and may reduce structural integrity at the donorsite. Furthermore, many patients complain of significant pain forseveral years after the donor surgery. Although, the supply of allograftmaterial is not so limited, allograft is also disadvantageous because ofthe risk of disease transmission and immune reactions. Furthermore,allogenic bone does not have the osteogenic potential of autogenous boneand therefore will incorporate more slowly and less extensively.

These disadvantages have led to the investigation of bioactivesubstances that regulate the complex cascade of cellular events of bonerepair. Such substances include bone morphogenetic proteins, for use asalternative or adjunctive graft materials. Bone morphogenetic proteins(BMPs), a class of osteoinductive factors from bone matrix, are capableof inducing bone formation when implanted in a fracture or surgical bonesite. Recombinantly produced human bone morphogenetic protein-2(rhBMP-2) has been demonstrated in several animal models to be effectivein regenerating bone in skeletal defects. The use of such proteins hasled to a need for appropriate carriers and fusion device designs.

SUMMARY OF THE INVENTION

In response to the needs still left unresolved by the prior devices, thepresent invention contemplates a hollow threaded interbody fusion deviceconfigured to restore the normal angular relation between adjacentvertebrae. In particular, the device includes an elongated body, taperedalong substantially its entire length, defining a hollow interior andhaving an outer diameter greater than the size of the space between theadjacent vertebrae. The body includes an outer surface with oppositetapered cylindrical portions and a pair of opposite flat tapered sidesurfaces between the cylindrical portions. Thus, at an end view, thefusion device gives the appearance of a cylindrical body in which thesides of the body have been truncated along a chord of the body's outerdiameter. The cylindrical portions are threaded for controlled insertionand engagement into the end plates of the adjacent vertebrae.

In another aspect of the invention, the outer surface is tapered alongits length at an angle corresponding, in one embodiment, to the normallordotic curvature of lower lumbar vertebrae. The outer surface is alsoprovided with a number of vascularization openings defined in the flatside surfaces, and a pair of elongated opposite bone ingrowth slotsdefined in the cylindrical portions. The bone ingrowth slots have atransverse width that is preferably about half of the effective width ofthe cylindrical portions within which the slots are defined.

In another embodiment, the interbody fusion device retains the sametapered configuration of the above embodiment, along with the truncatedside walls and interrupted external threads. However, in thisembodiment, the implant is not hollow but is instead solid. Boneingrowth is achieved by forming the solid tapered implant of a poroushigh strength material that permits bone ingrowth into interconnectedpores while retaining sufficient material for structural stability insitu. In one preferred embodiment, the material is a porous tantalumcomposite.

In another aspect of this invention, a hollow interbody fusion device isprovided with an osteogenic material to optimize fusion. The osteogenicmaterial comprises an osteoinductive protein in a suitable carrier.

In still another embodiment, the interbody fusion device is solidinstead of hollow and is composed of a porous high strength materialthat permits bone ingrowth into interconnected pores. In one preferredembodiment, the material is coated with an osteoinductive material.

In another aspect a cap is provided which securely blocks the opening ina fusion device to prevent expulsion of an osteogenic material fromwithin the device. The cap includes an occlusion body for blocking theopening and an elongated anchor for securing the occlusion body withinthe opening. In some embodiments the anchor includes a lip which isengageable to openings in the body wall.

In still another embodiment a tool is provided for manipulating caps forinterbody fusion devices. In one embodiment the tool includes a pair ofprongs each having facing engagement surfaces for engaging the fusiondevice, and a shaft slidably disposed between the prongs. The shaft hasa cap-engaging tip for engaging a tool hole in the cap. The prongsinclude a pair of releasing members on each of the facing engagementsurfaces. The releasing members have a height and a width for beinginsertable into apertures in a body wall in the fusion device todisengage elongate anchors of the cap from the apertures.

DESCRIPTION OF THE FIGURES

FIG. 1 is a side-elevational view in the sagittal plane of a fusiondevice of the prior art.

FIG. 2 is an enlarged perspective view of an interbody fusion deviceaccording to one embodiment of the present invention.

FIG. 3 is a side cross-sectional view of the interbody fusion deviceshown in FIG. 2, taken along line 3-3 as viewed in the direction of thearrows.

FIG. 4 is an end elevational view from the anterior end of the interbodyfusion device shown in FIG. 2.

FIG. 5 is a top-elevational view of the interbody fusion device shown inFIG. 2.

FIG. 6 is an A-P lateral view from the anterior aspect of the spineshowing two interbody fusion devices according to FIG. 2 implantedwithin the interbody space between L4 and L5.

FIG. 7 is a sagittal plane view of the interbody fusion device implantedbetween L4 and L5 shown in FIG. 6.

FIG. 8 is a perspective view of an alternative embodiment of theinterbody fusion device according to the present invention.

FIG. 8A is a perspective view of another embodiment of a taperedinterbody fusion device according to the present invention.

FIG. 9 is a top-elevational view of an implant driver according toanother aspect of the present invention.

FIG. 10 is an enlarged perspective view of the end of the implant driverengaged about an interbody fusion device, as depicted in FIG. 2.

FIG. 11 is an enlarged partial side cross-sectional view showing theimplant driver engaging the interbody fusion device, as shown in FIG.10.

FIG. 12 is an enlarged partial side cross-sectional view showing animplant driver of an alternative embodiment adapted for engaging theinterbody fusion device 10.

FIGS. 13(a)-13(d) show four steps of a method in accordance with oneaspect of the invention for implanting the interbody fusion device, suchas the device shown in FIG. 2.

FIGS. 14(a)-14(d) depict steps of an alternative method for implantingthe interbody fusion device, such as the device shown in FIG. 2.

FIG. 15 is an enlarged perspective view of an interbody fusion devicehaving an osteogenic material in the hollow interior according to oneembodiment of the present invention.

FIG. 16 is an end elevational view of the interbody fusion device shownin FIG. 15.

FIG. 17 is a perspective view of a cap according to this invention.

FIG. 18 is a side perspective view of a fusion device of this inventionwith the cap depicted in FIG. 17.

FIG. 19 is an elevational view of a cap manipulating tool of thisinvention.

FIG. 20 is a side elevational view of the tool depicted in FIG. 19.

FIG. 21 is an enlarged view of a portion of the tool of FIG. 19.

FIG. 22 is an elevational view of the tool of FIG. 19 engaged to a cap.

FIG. 23 is a side elevational view of the tool of FIG. 19 in a retractedposition.

FIG. 24 is a side elevational view of the tool of FIG. 19 in an extendedposition.

FIG. 25 is a partial cross-sectional view of the tool of FIG. 19.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

For the purposes of promoting an understanding of the principles of theinvention, reference will now be made to the embodiments illustrated inthe drawings and specific language will be used to describe the same. Itwill nevertheless be understood that no limitation of the scope of theinvention is thereby intended, such alterations and furthermodifications in the illustrated device, and such further applicationsof the principles of the invention as illustrated therein beingcontemplated as would normally occur to one skilled in the art to whichthe invention relates.

An interbody fusion device 10 in accordance with one aspect of thepresent invention is shown in FIGS. 2-5. The device is formed by asolid, conical, load bearing body 11, that is preferably formed of abiocompatible or inert material. For example, the body 11 can be made ofa medical grade stainless steel or titanium, or other suitable materialhaving adequate strength characteristics set forth herein. The devicemay also be composed of a biocompatible porous material, such as aporous tantalum composite provided by Implex Corp. For purposes ofreference, the device 10 has an anterior end 12 and a posterior end 13,which correspond to the anatomic position of the device 10 whenimplanted in the intradiscal space. The conical body 11 defines achamber or hollow interior 15 which is bounded by a body wall 16 andclosed at the posterior end 13 by an end wall 17 (see FIG. 3). Thehollow interior 15 of the device 10 is configured to receive autograftbone or a bone substitute material adapted to promote a solid fusionbetween adjacent vertebrae and across the intradiscal space.

In accordance with the invention, the interbody fusion device 10 is athreaded device configured to be screw threaded into the end plates ofthe adjacent vertebrae. In one embodiment of the invention, the conicalbody 11 defines a series of interrupted external threads 18 and acomplete thread 19 at the leading end of the implant. The completethread 19 serves as a “starter” thread for screwing the implant into thevertebral endplates at the intradiscal space. The threads 18 and 19 cantake several forms known in the art for engagement into vertebral bone.For instance, the threads can have a triangular cross-section or atruncated triangular cross-section. Preferably, the threads have aheight of 1.0 mm (0.039 in) in order to provide adequate purchase in thevertebral bone so that the fusion device 10 is not driven out of theintradiscal space by the high loads experienced by the spine. The threadpitch in certain specific embodiments can be 2.3 mm (0.091 in) or 3.0 mm(0.118 in), depending upon the vertebral level at which the device 10 isto be implanted and the amount of thread engagement necessary to holdthe implant in position.

In one aspect of the invention, the conical body 11, and particularlythe body wall 16, includes parallel truncated side walls 22, shown mostclearly in FIG. 4. The side walls are preferably flat to facilitateinsertion of the fusion device between the end plates of adjacentvertebrae and provide area between for bony fusion. The truncated sidewalls extend from the anterior end 12 of the device up to the completethreads 19 at the posterior end 13. Thus, with the truncated side walls22, the device 10 gives the appearance at its end view of an incompletecircle in which the sides are cut across a chord of the circle. In onespecific example, the interbody fusion device 10 has a diameter at itsanterior end of 16.0 mm (0.630 in). In this specific embodiment, thetruncated side walls 22 are formed along parallel chord linesapproximately 12.0 mm (0.472 in) apart, so that the removed arc portionof the circle roughly subtends 900 at each side of the device. Otherbenefits and advantages provided by the truncated side walls 22 of thefusion device 10 will be described in more detail herein.

To promote fusion, the devices of this invention may be provided withapertures defined through the body wall 16. The device 10 depicted inFIGS. 2-5 includes two types of body wall apertures, vascularizationopenings 24, 25 and bone ingrowth slots 27 as described below.

The conical body 11 of the device 10 includes a pair of vascularizationopenings 24 and 25 defined through each of the truncated side walls 22.These openings 24 and 25 are adapted to be oriented in a lateraldirection or facing the sagittal plane when the fusion device isimplanted within the intradiscal space. The openings are intended toprovide a passageway for vascularization to occur between the boneimplant material within the hollow interior 15 and the surroundingtissue. In addition, some bone ingrowth may also occur through theseopenings. The openings 24 and 25 have been sized to provide optimumpassage for vascularization to occur, while still retaining asignificant amount of structure in the conical body 11 to support thehigh axial loads passing across the intradiscal space between adjacentvertebrae.

The conical body 11 also defines opposite bone ingrowth slots 27, eachof which are oriented at 90° to the truncated side walls 22. Preferably,these slots 27 are directly adjacent the vertebral end plates when thedevice 10 is implanted. More particularly, as the threads 18 and 19 ofthe device are screwed into the vertebral endplates, the vertebral bonewill extend partially into the slots 27 to contact bone implant materialcontained within the hollow interior 15 of the device 10. As shown moreclearly in FIG. 5, the bone ingrowth slots 27 are configured to providemaximum opening for bone ingrowth, in order to ensure completearthrodesis and a solid fusion. Preferably, the slots have a lateralwidth that approximates the effective width of the threaded portions ofthe body.

Smaller apertures can lead to pseudo-arthrosis and the generation offibrous tissue. Since the bone ingrowth slots 27 of the presentinvention are directly facing the vertebrae, they are not situated in aportion of the device that must bear high loads. Instead, the truncatedside walls 22 will bear most of the load passing between the vertebralend plates through the interrupted threads 18 and across the intradiscalspace.

In a further feature, the anterior end 12 of the body wall 16 can definea pair of diametrically opposed notches 29, which are configured toengage an implant driver tool as described herein. Moreover, the endwall 17 at the posterior end 13 of the implant can be provided with atool engagement feature (not shown). For example, a hex recess can beprovided to accommodate a hex driver tool, as described further herein.

In one important feature of the interbody fusion device of the presentinvention, the body 11 includes a tapered or conical form. In otherwords, the outer diameter of the device at its anterior end 12 is largerthan the outer diameter at the posterior end 13. As depicted in FIG. 3,the body wall 16 tapers at an angle A about the centerline CL of thedevice 10. The taper of the body wall 16 is adapted to restore thenormal relative angle between adjacent vertebrae. For example, in thelumbar region, the angle A is adapted to restore the normal lordoticangle and curvature of the spine in that region. In one specificexample, the angle A is 8.794°. It is understood that the implant mayhave non-tapered portions, provided that the portions do not otherwiseinterfere with the function of the tapered body.

The taper angle A of the implant, coupled with the outer diameter at theanterior and posterior ends of the fusion device 10, define the amountof angular spreading that will occur between the adjacent vertebrae asthe implant is placed or screwed into position. This feature is depictedmore clearly in FIGS. 6 and 7 in which a preferred construct employing apair of fusion devices 10 is shown. In the depicted construct, thedevices 10 are disposed between the lower lumbar vertebrae L4 and L5,with the threads 18 and 19 threaded into the end plates E of the twovertebrae. As shown in FIG. 7, as the device 10 is threaded into the endplates E, it advances in the direction of the arrow I toward the pivotaxis P of the vertebral level. The pivot axis P is nominally the centerof relative rotation between the adjacent vertebrae of the motionsegment. As the tapered fusion device 10 is driven further in thedirection of the arrow I toward the pivot axis P, the adjacent vertebraeL4 and L5 are angularly spread in the direction of the arrows S. Depthof insertion of the fusion device 10 will determine the ultimatelordotic angle L achieved between the two vertebrae.

In specific embodiments of the implant 10, the outer diameter or threadcrest diameter at the anterior end 12 can be 16, 18 or 20 mm, and theoverall length of the device 26 mm. The sizing of the device is drivenby the vertebral level into which the device is implanted and the amountof angle that must be developed.

In another aspect of the invention, device 10 is sized so that two suchcylindrical bodies 11 can be implanted into a single disc space, asshown in FIG. 6. This permits the placement of additional bone graftmaterial between and around the devices 10 in situ. This aspect furtherpromotes fusion across the intradiscal space and also serves to morefirmly anchor the devices between the adjacent vertebrae to preventexpulsion due to the high axial loads at the particular vertebral level.

In one specific embodiment of the interbody fusion device 10, thevascularization opening 24 is generally rectangular in shape havingdimensions of 6.0 mm (0.236 in) by 7.0 mm (0.276 in). Similarly, thevascularization opening 25 is rectangular with dimensions of 4.0 mm(0.157 in) by 5.0 mm (0197 in). Naturally, this opening is smallerbecause it is disposed at the smaller posterior end 13 of the device 10.The bone ingrowth slots 27 are also rectangular in shape with a longdimension of 20.0 mm (0.787 in) and a width of 6.0 mm (0.236 in). It hasbeen found that these dimensions of the vascularization openings 24, 25and slots 27 provide optimum bone ingrowth and vascularization. Inaddition, these openings are not so large that they compromise thestructural integrity of the device or that they permit the bone graftmaterial contained within the hollow interior 15 to be easily expelledduring implantation.

As can be seen in FIG. 7, when the device is in position between the L4and L5 vertebrae, the vascularization openings 24 and 25 are side facingto contact the highly vascularized tissue surrounding the vertebrae. Inaddition, as can be seen in FIG. 6, the bone ingrowth slots 27 areaxially directed so that they contact the vertebral end plates E.

In an alternative embodiment of the invention, shown in FIG. 8, aninterbody fusion device 36 is formed of a conical, load bearing body 31.The body wall 34 defines a chamber or hollow interior 33 as with thefusion device 10 of the previous embodiment. However, in this embodimentthe truncated side wall 38 does not include any vascularizationopenings. Moreover, the bone ingrowth slots 39 on opposite sides of thedevice 30 are smaller. This means that the interrupted threads 36 on theexterior of the device 30 extend a greater length around the implant.Such a design could be utilized if a porous material (e.g., a poroustantalum composite) were used to provide additional surface area fortissue ingrowth and anchorage to the adjacent bone or if a bone growthpromoting protein were used to increase the fusion rate. Also, thisinterbody fusion device 30 of the embodiment shown in FIG. 8 can haveapplication at certain vertebral levels where the risk of expulsion ofthe device is greatest. Consequently, the amount of thread contact isincreased to prevent such expulsion. Prior to insertion, the hollowinterior 15 of the fusion device 10 is filled completely with bone orsubstitute to facilitate this pre-loading.

In a further embodiment using a porous material, the interbody fusiondevice 110 of FIG. 8A retains the tapered configuration of the previousembodiments, but is solid instead of hollow. The device 110 comprises atapered, load bearing body 111 having a larger outer diameter at isanterior end 112 than at is posterior end 113. The entire body 111 issolid leaving a closed surface, such as surface 115, at both ends of theimplant. The device includes the interrupted threads 118, starterthreads 119 and truncated side walls 122 of the prior embodiments. Adriving tool slot 129 can also be defined in the end surface 115.Alternatively, the starter threads 119 can be eliminated leaving anunthreaded cylindrical portion at the posterior end of the implant.Similarly, the driving tool slot 129 take on many configurationsdepending upon the design of the tool used to insert the device 110 intothe intradiscal space.

The benefits of the embodiment of the fusion device shown in FIG. 8A areespecially appreciated by the use of a porous, high strength material toform the solid body 111. In the preferred embodiment, this material is aporous tantalum-carbon composite marketed by Implex Corp. under thetradename HEDROCEL® and described in U.S. Pat. No. 5,282,861 to Kaplan,which description is incorporated herein by reference. Due to the natureof the HEDROCEL® material, the entire exterior surface of the solid body111 includes pores 130 that are interconnected throughout the body. Thesubstrate of the HEDROCEL® carbon-tantalum composite is a skeleton ofvitreous carbon, or a reticulated open cell carbon foam, which defines anetwork of interconnecting pores. The substrate is infiltrated withvapor-deposited thin film of a metallic material. The metallic materialis preferably a Group VB transition metal such as tantalum, niobium oralloys thereof.

HEDROCEL® is preferred because it provides the advantages of both metaland ceramic implants without the corresponding disadvantages. HEDROCEL®is well suited for the interbody fusion device of the present inventionbecause it mimics the structure of bone and has a modulus of elasticitythat approximates that of human bone. The interconnected porosityencourages bone ingrowth and eliminates dead ends which limitvascularization of the bone. The infiltrated metal film providesstrength and stiffness without significant weight increase. A HEDROCEL®implant is sufficiently strong to maintain the intervertebral space andnormal curvature of the spine at the instrumented motion segment. At thesame time, stress shielding is avoided. This composite material is alsoadvantageous because it eliminates the need for allografts orautografts.

On additional advantage of this material is that it does not undergoresorption. This prevents early degradation which can inhibit boneregeneration. A non-resorbable implant is also beneficial where completebone ingrowth may not be achieved. Disadvantages of permanent,non-resorbable implants, however, are avoided because of the excellentbiocompatibility and osteoconductivity of the composite.

While HEDROCEL® is preferred, it is contemplated that any suitable highstrength porous material may be used. For example, ceramics could beused, such as alumina, zirconia, silicone nitride, carbon, glass, coral,hydroxyapatite, calcium sulfate, ferric calcium phosphorous oxide, zinccalcium phosphorous oxide, calcium phosphate and calcium aluminateceramics. It is contemplated that calcium phosphate compositions, suchas hydroxyapatite, tricalcium phosphate and biphasic ceramics thereof,could be employed if the material could be manufactured to withstand thehigh spinal loads.

Other metal-open-celled substrate composites are also contemplated. Forexample, the substrate may be other carbonaceous materials, such asgraphite, or ceramics, such as tricalcium phosphate or calciumaluminate. Any suitable metal is contemplated, but Group VB elements,such as tantalum and niobium, and their alloys, are preferred. Tantalumis particularly preferred for its good mechanical properties andbiocompatibility.

The interbody fusion devices of this invention can be implanted using animplant driver 50, shown in FIG. 9, according to one aspect of theinvention. The implant driver 50 is comprised of a shaft 51 and sleeve52 concentrically disposed about the shaft. Tongs 54 are formed at oneend of the shaft for gripping the interbody fusion device 10 forimplantation. The tongs include a tapered outer surface 55 and anopposite flat inner surface 56 adapted to engage the truncated sidewalls 22 of the interbody fusion device. The tapered outer surface 55conforms to the root diameter of the interrupted threads 18 so that thetongs 54 essentially complete the full cylindrical shape of the bodywall 16. The adaptation of the tong's tapered outer surface 55facilitates screw insertion of the interbody fusion device 10 since theouter surface 55 will ride within the tapped bore in the vertebralendplates.

Each of the tongs is provided with interlocking fingers 58 and a drivingprojection 59 extending from the inner surface 56. The function of thesecomponents is shown more clearly with reference to FIG. 11. Referringfirst to FIG. 9, the shaft 51 defines a hinge slot 62 supporting each ofthe pair of tongs 54. The hinge slot 62 is configured so that the tongswill have a naturally biased position spread sufficiently apart toaccept the tapered interbody fusion device 10 therebetween. The shaft 51defines a conical taper 63 between the hinged slot 62 and each of thetongs 54. This conical taper mates with a conical chamfer 67 defined onthe inner wall of the sleeve 52. Thus, as the sleeve 52 is advancedtoward the tongs 54, the conical chamfer 67 rides against the conicaltaper 63 to close or compress the hinge slot 62. In this manner, thetongs 54 are pushed toward each other and pressed into grippingengagement with the interbody fusion device situated between the tongs.

The shaft 51 and sleeve 52 are provided with a threaded interface 65which permits the sleeve 52 to be threaded up and down the length of theshaft. Specifically, the threaded interface 65 includes external threadson the shaft 51 and internal threads on the sleeve 52 having the samepitch so that the sleeve can be readily moved up and down the implantdriver 50. The shaft 51 is also provided with a pair of stops 69 whichrestrict the backward movement of the sleeve 52 to only the extentnecessary to allow the tongs 54 to separate a sufficient distance toaccept the interbody fusion device 10.

The use of the implant driver 50 is shown with reference to FIGS. 10 and11. As can be seen in FIG. 10, the outer surface 55 of the tongs 54reside generally flush with the root diameter of the interrupted threads18. As seen in FIG. 11, the interlocking fingers 58 can be arranged tofit within the vascularization opening 24 on each of the truncated sidewalls 22. In a similar fashion, the driving projections 59 engage thedriving tool slots 29 at the anterior end 12 of the conical body 11. Thecombination of the interlocking fingers 58 and driving projections 59firmly engage the interbody fusion device 10 so that the device can bescrew threaded into a tapped or untapped opening in the vertebral bone.

An alternative embodiment of the implant driver is shown in FIG. 12. Thedriver 90 includes a shaft 91, having a length sufficient to reach intothe intradiscal space from outside the patient. Connected to the end ofshaft 91 is a head which defines a pair of opposite tongs 93, each ofwhich are configured for flush contact with the flat truncated sidewalls 22 of the fusion device 10. Like the tongs 54 of the previouslydescribed implant driver 50, the outer surface of the tongs iscylindrical to correspond to the cylindrical threaded portion of thedevice.

Unlike the implant driver 50, the driver 90 of the embodiment in FIG. 12uses an expanding collet assembly to firmly grip the fusion device 10for insertion into the body. Specifically, the head 92 defines a collet94 having a central collet bore 95 formed therethrough. The collet 94terminates in an annular flange 96 that at least initially has adiameter slightly smaller than the inner diameter of the fusion device10 at its end 12. An expander shaft 97 slidably extends through thecollet bore and includes a flared tip 98 situated adjacent and extendingjust beyond the annular flange 96. The flared tip 98 of the expandershaft 97 starts at a diameter sized to slide within the collet bore 95and gradually flares to a diameter larger than the bore.

The implant driver 90 includes a puller shaft 99 slidably disposedwithin a bore 100 defined in the shaft 91. The puller shaft 99 has alocking chamber 101 at its end which engages a locking hub 102 formed atthe end of the expander shaft 97. The puller shaft 99 projects beyondthe end of shaft 91 for access by the surgeon. When the puller shaft 99is pulled, it pulls the expander shaft 97 away from the annular flange96 of the collet 94 so that the flared tip 98 becomes progressivelyengaged within the collet bore 95. As the tip 98 advances further intothe bore 95, the annular flange 96 expands from its initial diameter toa larger second diameter sufficient for firm gripping contact with theinterior of the fusion device 10. With the fusion device so engaged, theimplant driver can be used to insert the device 10 into the surgicalsite, after which the expander shaft can be advanced beyond the colletbore to release the flared tip and, consequently, the fusion device.

In accordance with the present invention, two methods for implanting theinterbody fusion device 10 are contemplated. First, with reference toFIGS. 13(a)-13(d), an anterior approach is shown. As a preliminary step,it is necessary to locate appropriate starting points for implanting thefusion device, preferably bilaterally. In the first step of the anteriorapproach, a dilator 75 is disposed between the vertebral endplates E todilate the disc space between the L4 and L5 vertebrae. (It isunderstood, of course, that this procedure can be applied at othervertebral levels). In the second step, shown in FIG. 13(b), an outersleeve 76 is disposed about the disc space. The outer sleeve 76 can beof a known design that is configured to positively engage the anterioraspect of the vertebral bodies to firmly, but temporarily, anchor theouter sleeve 76 in position. In essence, this outer sleeve 76 operatesas a working channel for this laproscopic-type approach. In this step ofFIG. 13(b), a drill 77 of known design is extended through the outersleeve and used to drill out circular openings in the adjacent vertebralbodies. The openings can be tapped to facilitate screw insertion of thefusion device, although this step is not necessary.

In the next step shown in FIG. 13(c), the fusion device 10 is engaged bythe implant driver 50 and extended through the outer sleeve 76 until thestarter thread 19 contacts the bone opening. The implant driver 50 canthen be used to screw thread the fusion device into the tapped oruntapped opening formed in the vertebral endplate E. It is understoodthat in this step, other suitable driving tools could be used, such as ascrew driver type device to engage the driving tool slots 29 at theanterior end 12 of the device 10. As discussed previously, the degree ofinsertion of the fusion device 10 determines the amount of lordosisadded or restored to the vertebral level. In the final step, the implantdriver is removed leaving the fusion device 10 in position. It can beseen that once implanted, the closed end wall 17 is directed toward theposterior aspect of the vertebrae. The hollow interior 15 is open at itsanterior end, but can be closed by a plastic or metal material, ifnecessary.

In a second inventive method, as depicted in FIGS. 14(a)-14(d), aposterior approach is implemented. The first two steps of the posteriorapproach are similar to that of the prior anterior approach, except thatthe dilator 75, outer sleeve 76 and drill 77 are introduced posteriorlyinto the instrumented region. This approach may require decorticationand removal of vertebral bone to accept the outer sleeve 76. In thethird step of this method, the fusion device 10 is inserted through theouter sleeve 76 into the dilated disc space. It is understood that thedisc space is dilated only to the extent necessary to receive theimplant with the truncated side walls 22 directly facing the vertebralendplates E. Thus, as shown in FIG. 14(c), the bone ingrowth slot 27 isfacing laterally, rather than coronally, as expected for its finalimplanted position. A suitable driving tool 80 can be provided toproject the fusion device 10 through the outer sleeve 76 and into theintradiscal space. In one embodiment, the driving tool 80 includes aprojection 81 which is configured to engage a slot opening formed in theend wall 17 at the posterior end 13 of the fusion device 10. An internalthread (not shown) can be used to fix the device 10 to the driver 80.

Once the fusion device 10 has been advanced into the intradiscal spaceto the appropriate depth relative to the pivot axis P of the vertebrae,the driving tool 80 is used to rotate the implant in the direction ofthe rotational arrow R in FIG. 14(c). As the driving tool 80 is rotated,the device itself rotates so that the interrupted threads 18 startcutting into the vertebral bone at the endplates E. In this manner, theimplant operates as a cam to separate the adjacent vertebrae in thedirection of the spreading direction arrows S in FIG. 14(c). Thiscamming approach provides a somewhat easier insertion procedure in thata single rotation is required to lock the implant into the vertebralbone. In contrast, the formerly discussed screw insertion techniquerequires continuous threading of the device into position.

With either technique, the position of the fusion device 10 with respectto the adjacent vertebrae can be verified by radiograph or othersuitable techniques for establishing the angular relationship betweenthe vertebrae. Alternatively, the preferred depth of insertion of theimplant can be determined in advance and measured from outside thepatient as the implant is positioned between the vertebrae.

It can be seen that the interbody fusion device 10, implant driver 50and techniques of the present invention provide significant advantagesover the prior devices and techniques. Specifically, the fusion device10 provides a hollow threaded implant that maximizes the potential forbony fusion between adjacent vertebrae, while maintaining the integrityof the implant itself. It is understood that the spine enduressignificant loads along its axial length, which loads must be supportedby the fusion device 10 at least until solid fusion is achieved. Thedevice 10 also provides means for vascularization and tissue ingrowth tooccur which speeds up the fusion rate and enhances the strength of theresulting fused bony mass. Another significant aspect is that thetapered shape of the implant allows the surgeon to restore and maintainthe proper curvature or relative angle between vertebral bodies. Thisavoids the significant problems associated with prior devices in whichproduct deformities arise and the spine goes out of balance. A furtheradvantage achieved by the device and its implant driver is thecapability for insertion either anteriorly or posteriorly using alaproscopic approach. Depending upon the vertebral level, eitherapproach may be preferred, so it is important that the implant beadapted for insertion from either direction. Controlled insertion of thedevice is provided by the screw-in technique used for anterior insertion(vs. pounding in) and for the slide-in and cam method used for theposterior technique.

During a surgical implantation procedure, the surgeon may apply anosteogenic material to a fusion device 10 or 30 by packing the hollowinterior 15 with an osteogenic material. Alternatively, in the case of afusion device such as device 30 or 110, the osteogenic material can beapplied by introducing an osteogenic composition to the pores of thebone ingrowth material. Any suitable osteogenic material or compositionis contemplated. The osteogenic compositions preferably comprise atherapeutically effective amount of a bone inductive factor such as abone morphogenetic protein in a pharmaceutically acceptable carrier.

For the osteogenic compositions, any suitable carrier which provides avehicle for introducing the osteogenic material into the pores of thebone ingrowth material or the hollow interior of the device iscontemplated. Such carriers are well known and commercially available.The choice of carrier material is based on biocompatibility,biodegradability, mechanical properties and interface properties. Theparticular application of the compositions of the invention will definethe appropriate formulation. The carrier may be any suitable carriercapable of delivering the proteins to the implant. Most preferably, thecarrier is capable of being resorbed into the body. One preferredcarrier is an absorbable collagen sponge marketed by IntegraLifeSciences Corporation under the trade name Helistat® AbsorbableCollagen Hemostatic Agent. Another preferred carrier is an open cellpolylactic acid polymer (OPLA). Other potential matrices for thecompositions may be biodegradable and chemically defined calciumsulfate, tricalcium phosphate (TCP), hydroxyapatite (HA), biphasicTCP/HA ceramic, polylactic acids and polyanhydrides. Other potentialmaterials are biodegradable and biologically well defined, such as boneor dermal collagen. Further matrices are comprised of pure proteins orextracellular matrix components. The osteoinductive material may also bean admixture of the osteoinductive cytokine and a polymeric acrylicester carrier. The polymeric acrylic ester can be polymethylmethacrylic.

For the hollow fusion devices, such as device 10 the carriers can beprovided in strips or sheets which may be folded to conform to thehollow interior 15 as shown in FIGS. 15 and 16. It may be preferable forthe carrier to extend out of openings of the devices, such as thevascularization openings 24, 25, to facilitate contact of the osteogenicmaterial with the highly vascularized tissue surrounding the vertebrae.In one embodiment, the osteogenic material 100 includes a polylacticacid polymer acting as a carrier for a bone morphogenetic protein, suchas BMP-2. In this specific embodiment, the osteogenic material 100 is inthe form of a sheet 101 that is overlapped at folds 102 within thehollow interior 15 of the device 10. Preferably, the sheet 101 is longenough so that when it is folded within the device 10 it substantiallycompletely fills the hollow interior and extends at least partially intothe vascularization openings 24 and 25.

As shown in FIGS. 15 and 16, the sheet 101 is folded generally parallelwith the truncated side walls 22 so that the folds 102 of the sheet 101are disposed adjacent the slots 27 in the threaded portion of thedevice. Alternatively, the sheet 101 can be folded so that the layersbetween the folds are generally perpendicular to the side walls 22. Inthis instance, the sheet 101 may extend at least partially into theslots 27.

The osteogenic material 100 can also be provided in several strips sizedto fit within the hollow interior 15 of the fusion device 10. The strips(not shown) can be placed one against another to fill the interior. Aswith the folded sheet 101, the strips can be arranged within the device10 in several orientations, such as with the surface of the stripsdirected either toward the vascularization openings 24, 25 or toward theslots 27. Preferably, the osteogenic material 100, whether provided in asingle folded sheet or in several overlapping strips, has a lengthcorresponding to the length of the hollow interior 15 of the device 10and a width corresponding to the width of the device transverse to itslongitudinal axis.

As discussed in the Kaplan patent, the open cell tantalum materialprovides highly interconnected three-dimensional porosity thatencourages bone ingrowth. Kaplan type materials facilitate bone ingrowththroughout the entire device for complete fusion and have the strengthof metal without the disadvantages of metal such as stress shielding andincomplete fusion. An additional benefit of the porosity of thesematerials is that a bone growth inducing composition can be introducedinto the pores. For example, in one embodiment, the composition includesa bone morphogenetic protein in a liquid carrier which can be introducedinto the pores to promote fusion. BMPs have been found to significantlyreduce the time required to achieve arthrodesis and fusion across aninstrumented disc space. Most preferably, the bone morphogenetic proteinis a BMP-2, such as recombinant human BMP-2. However, any bonemorphogenetic protein is contemplated including bone morphogeneticproteins designated as BMP-1 through BMP-13. BMPs are commerciallyavailable from Genetics Institute, Inc., Cambridge, Mass. and may alsobe prepared by one skilled in the art as described in U.S. Pat. No.5,187,076 to Wozney et al.; U.S. Pat. No. 5,366,875 to Wozney et al.;U.S. Pat. No. 4,877,864 to Wang et al.; U.S. Pat. No. 5,108,922 to Wanget al.; U.S. Pat. No. 5,116,738 to Wang et al.; U.S. Pat. No. 5,013,649to Wang et al.; U.S. Pat. No. 5,106,748 to Wozney et al.; and PCT PatentNos. WO93/00432 to Wozney et al.; WO94/26893 to Celeste et al.; andWO94/26892 to Celeste et al.

The BMP may be provided in freeze-dried form and reconstituted insterile water or another suitable medium or carrier. The carrier may beany suitable medium capable of delivering the proteins to the implant.Preferably the medium is supplemented with a buffer solution as is knownin the art. The bone growth inducing composition can be introduced intothe pores in any suitable manner. For example, the composition may beinjected into the pores of the implant. In other embodiments, thecomposition is dripped onto the biocompatible material or thebiocompatible material is soaked in the composition. In one specificembodiment of the invention, rhBMP-2 is suspended or admixed in a liquidcarrier, such as water or liquid collagen. The liquid can be drippedinto the device or the device can be immersed in a suitable quantity ofthe liquid, in either case for a period of time sufficient to allow theliquid to invade all of the interconnected pores throughout the porematerial of the device.

In some cases, a BMP-bonding agent is applied to the porousbiocompatible material of the implant prior to introduction of the BMPso that the agent can coat the pores of the device. Preferably, theagent is a calcium phosphate composition. It has been discovered thatthe rate of delivery of bone morphogenetic proteins to the fusion sitecan be controlled by the use of such agents. The calcium phosphatecompositions are thought to bond with the bone morphogenetic protein andprevent the BMP from prematurely dissipating from the device beforefusion can occur. It is further believed that retention of the BMP bythe agent permits the BMP to leach out of the device at a rate that isconducive to complete and rapid bone formation and ultimately, fusionacross the disc space. Any suitable, biocompatible calcium phosphatecomposition is contemplated. In a preferred embodiment, a layer ofhydroxyapatite several microns thick is applied to the Kaplan material.The hydroxyapatite covers the tantalum film-covered ligaments whileleaving the pores open. Also contemplated are tricalcium phosphateceramics and hydroxyapatite/tricalcium phosphate ceramics.

The calcium phosphate composition may be applied to the porousbiocompatible material of the implant in any suitable manner such asplasma spraying or chemical dipping where the porous material is dippedinto a slurry of calcium phosphate composition. Methods for applying acoating of calcium phosphate compositions are described in thefollowing: U.S. Pat. No. 5,164,187 to Constantz et al., U.S. Pat. No.5,030,474 to Saita et al, U.S. Pat. No. 5,330,826 to Taylor et al, U.S.Pat. No. 5,128,169 to Saita et al, Re. 34,037 to Inoue et al, U.S. Pat.No. 5,068,122 to Kokubo et al, and U.S. Pat. Nos. 5,188,670 and5,279,831 to Constantz which are hereby incorporated by reference.

For hollow spacers, such as the one depicted in FIG. 2, this inventionprovides a cap 300 (FIG. 17) for blocking the opening 15 a to preventexpulsion of graft material within the chamber 15. (See FIG. 18.) Inpreferred embodiments, the cap 300 includes an occlusion body 301 sizedand shaped for contacting and closing the opening 15 a and an elongateprong or anchor 310 projecting from the body 301.

In the embodiment shown in FIG. 17, the occlusion body 301 includes anouter wall 304, an opposite inner surface 306 and a flange 307 incommunication with and connected to the outer wall 304. The flange 307defines an engaging surface 308 for contacting the internal surface ofthe body wall 16 of the load bearing body 11′. The flange 307 alsoprevents the cap 300 from traveling into the interior of the fusiondevice.

The anchor 310 includes a first end 311 attached to the occlusion body301 and an opposite second end 312 having engaging means for engagingthe load bearing body 11′ to hold the occlusion body 301 within theopening 15 a. In a preferred embodiment, the engaging means is a lip 315projecting from the second end 312 which contacts the internal surfaceof the load bearing body 11′. Preferably the anchor 310 has a length lwhich reaches from the occlusion body 301 to a body wall aperture whenthe cap 300 is inserted into the opening 15 a. In FIG. 18, the lip 315is engaged to a vascularization opening 24′. In some embodiments, theouter wall 304 of the cap 300 will preferably be flush or nearly flushwith the opening 15 a as shown in FIG. 18 for a low profile device.

The cap 300 shown in FIG. 17 also includes a second, opposite elongateanchor 325 projecting from the occlusion body 301. It is of coursecontemplated that any number of anchors could be provided. The anchorsare preferably composed of a resilient material, particularly when morethan one anchor is provided. The resilient material allows the anchors310, 325 to be slightly deflected by an inward force F for insertion.Once the cap 300 is inserted into the opening 15 a the force on theanchors 310, 325 is released allowing the anchors 310, 325 to return totheir normal configuration in which the anchors 310 engage the loadbearing body 11′.

Any suitable material is contemplated for the caps of this invention,such as biocompatible metals and polymers. In one preferred embodiment,the cap is composed of titanium. In another preferred embodiment the capis polymer, such as for example, polyethylene, polyvinylchloride,polypropylene, polymethylmethacrylate, polystyrene and copolymersthereof, polyesters, polyamides, fluorocarbon polymers, rubbers,polyurethanes, polyacetals, polysulfones and polycarbonates.Biodegradable polymers, including, for example, glycolide, lactide andpolycarbonate based polymers, are also contemplated for the cap. Suchpolymers could be manufactured to degrade after the expectedincorporation/degradation of the graft material or graft substitute.Polyethylene is particularly preferred because it is inert and providesa smooth, nonirritating surface. Another benefit is that polyethylene isradiolucent and does not interfere with radiological visualization.Other suitable materials include stainless steel and HEDROCEL®.

The cap also preferably includes osteogenic apertures 305 definedthrough the outer wall 304 which are sized to permit bone ingrowth andprotein egress. The osteogenic apertures 305 are particularly preferredwhen a material such as polyethylene is chosen for the cap. Suchbiocompatible polymers are not known to allow bony attachments as doother materials such as titanium. Therefore, a solid plastic cap couldimpede bone formation in the area of the cap. The osteogenic aperturesare also advantageous because they facilitate controlled diffusion ofbone growth proteins implanted within the chamber to facilitate bonybridging and fusion around the device. The resulting fusion around thedevice supplements the device ingrowth fusion mass within the device fora more solid overall fusion. The bony bridging around a device is alsofavorable because it serves as a better indicator of the success of theprocedure. Bone ingrowth within a device is difficult to assess usingplain film radiographs but bony bridging outside a device can be easilyvisualized.

Any suitably sized cap is contemplated. The dimensions of the caps willvary as needed to effectively block the openings of fusion devices.Referring now to FIG. 17, one cap has a length L (of the occlusion bodyincluding the flange) of 0.548 inches (13.7 mm), a length L′ of theocclusion body without the flange of 0.488 inches (12 mm), a width W of0.330 inches (8.25 mm) and a height H of 0.377 inches (9.4 mm).

This invention also provides tools for manipulating caps for interbodyfusion devices. The tools include means for engaging the cap and meansfor engaging the fusion device for inserting and removing a cap. Duringa surgical procedure, the cap 300 could be inserted into the opening 15a after the fusion device 10′ is implanted and the chamber is packedwith osteogenic material. In some cases it may be necessary to remove acap during or after the surgery to replace or remove the osteogenicmaterial in the chamber or to access the fusion device for revision. Thecap 300 shown in FIG. 17 includes a tool hole 320 for receiving aninsertion or removal tool. The hole 320 is preferably threaded but anysuitable engagement surface, such as an internal hex or the like, iscontemplated.

One embodiment of a tool 400 of this invention is depicted in FIGS. 19and 20. The tool 400 includes a pair of prongs 401 each having aproximal end 402 defining first engaging means for engaging the fusiondevice and a shaft 410 having a first end 411 defining second engagingmeans for engaging a cap. The tool also includes means for slidablysupporting the shaft 410 between the prongs 401. In one embodiment, theinvention includes a body or housing 420 defining a passageway 421therethrough. The distal end 403 of the prongs 401 are attached to thehousing 420 in this embodiment. As depicted in FIG. 20, the prongs 401can be attached to the housing 420 with screws 404. Of course anysuitable fastening means is contemplated.

The prongs 401 can be used to steady the fusion device for insertion ofthe cap or can be used to engage the fusion device and/or the cap forremoval of the cap. In the embodiment depicted in FIG. 19, the proximalend 402 of the prongs 401 includes facing engagement surfaces 404 forengaging the fusion device. In a most preferred embodiment, a pair ofreleasing members 405 are disposed on each of the facing engagementsurfaces 404. Referring now in particular to FIG. 21, the releasingmembers 405 have a height h and a width w for being insertable intoapertures 24′ in a fusion device 10′. The tool of FIGS. 19-21 can beused to remove a cap 300 of this invention which is inserted into theopening 15 a of a fusion device 10′ as shown in FIG. 18. The releasingmembers 405 are insertable into the apertures 24′ for applying pressureF to elongate arms or anchors 310 of the cap 300 to deflect the anchors310 inwardly to release the cap 300 from the interbody fusion device10′. In embodiments where the anchors 310 include a lip 315 or otherengaging means, the releasing members 405 are insertable into theapertures 24′ to disengage the lips 315 from the apertures.

The distance d between the proximal ends 402 of the prongs 401 ispreferably adjustable to facilitate engaging portions of the fusiondevice and/or cap. In a preferred embodiment this is accomplished bycomposing the prongs 401 of a resilient material such as stainlesssteel. The adjustable feature could be obtained by other means such asby providing a hinge at the distal end 403 of the prongs 401. Any othersuch suitable means of adjusting the distance d are contemplated.

Referring again to FIG. 19, the first end 411 of the shaft 410 defines acap-engaging tip 415 configured for matingly engaging a tool hole in thecap. In the embodiment shown in FIG. 19, the cap engaging tip 415defines threads for engaging a threaded tool hole in a cap 300 as shownin FIG. 22. Any suitable tool engaging means is contemplated such as,for example, a hex for engaging an internal hex in a cap.

In the embodiment shown in FIGS. 19-22, the shaft 410 is slidablydisposed within the passageway 421 of the housing 420. The shaft 410 isslidable between a retracted position (FIG. 23) and an extended position(FIG. 24) at which the first end 411 is adjacent and between theproximal ends 402 of the prongs 401. To insert a cap into a fusiondevice, the prongs 401 can be used to engage and hold the fusion device.The engaging end 415 engages a tool hole of the cap and the cap isdelivered to the fusion device by sliding the shaft 410 to the extendedposition (FIG. 24). Where the engaging end 415 is threaded, the shaft410 is unscrewed from the cap by rotating the shaft 410 within thehousing 420 after the cap is inserted into the fusion device. To removea cap, the prongs 401 are first engaged to the fusion device. The prongs401 may engage a body wall of the device. When used with a cap 300 suchas depicted in FIGS. 17 and 18, the releasing members 405 are insertedinto the apertures 24′ to disengage the lips 315 and deflect the anchors310, 325 inwardly. The shaft 410 is then moved from the retractedposition (FIG. 23) to the extended position (FIG. 24) and then rotatedto engage the tool engaging hole 320 of the cap 300. The shaft is thenreturned to the retracted position (FIG. 23) with the cap 300 engaged tothe engaging end 415.

In the embodiment depicted in FIG. 19 the first end 411 of the shaft 410is a metal rod 412 attached to an autoclavable plastic center rod 413.An autoclavable plastic is chosen for a light weight yet reusabledevice. In one embodiment, the metal rod 412 is press fit into theplastic center rod and is further engaged by a pin 414.

In one embodiment the center rod 413 of the shaft 410 is slip fit intothe passageway 421 of the housing 420. Proximal and distal stop membersare preferably provided to prevent the shaft 410 from sliding out of thehousing 420. A proximal stop member is preferably disposed on the centerrod 413 adjacent the first end 411 for preventing the first end 411 fromentering the passageway 421. As shown in FIG. 19, the proximal stopmember is an O-ring 430 engaged to the center rod 413 of the shaft 410.In one embodiment, the center rod 413 defines a groove 431 (FIG. 25) forseating the O-ring 430. The groove 431 is positioned so that when anO-ring 430 is seated therein the shaft 410 cannot move beyond theretracted position shown in FIG. 23 to prevent the first end 411 fromentering the passageway 421.

A distal stop member 440 may be attached to the second end 416 of theshaft 410 which has a perimeter that is larger than a perimeter of thepassageway 421 to prevent the second end 416 from entering thepassageway 421. As shown in FIG. 25, where the stop member 440 andpassageway 421 are circular, the distal stop member 440 has a diameterD₁ which is larger than a diameter D₂ of the passageway 421.

The tools of this invention are also preferably provided with a distalshaft manipulating member attached to the second end 416 of the shaft410 for rotating and sliding the shaft 410 within the passageway 421. Inthe embodiment shown in FIG. 19 the manipulating member is thumb wheel441. Thumb wheel 441 has a dimension or diameter D₁ that is larger thandiameter D₂ and therefore also is the distal stop member 440.

To promote a further understanding and appreciation of the invention,the following specific examples are provided. These examples areillustrative of the invention and should in no way be construed aslimiting in nature.

EXAMPLE 1

Surgical Technique: Twenty-one mature female Alpine goats were used inthis study. The goats weighed between 42 and 62 kilograms. All the goatsunderwent a surgical procedure under general endotracheal anesthesiausing intravenous valium and ketamine for induction, and inhalationhalothane for maintenance anesthesia. The anterior neck was prepped in asterile fashion and a right anterolateral approach to the cervical spinewas carried out through a longitudinal skin incision. The well developedlongus coli muscle was incised in the midline, and the disc spaces atC2-C3, C3-C4, and C4-C5 exposed. Anterior cervical discectomies werecarried out at each level by first excising the soft disc. An 8 mmdistraction plug centered on a post was then tapped into the disc spaceproviding distraction of the space. A working tube was then passed overthe post and prongs at the end of the tube tapped into the vertebralbodies above and below the disc space. These prongs maintaineddistraction of the disc space as the centering post and distraction plugwere removed. The disc space and vertebral bodies/endplates were thenreamed with a 10 mm reamer through the working tube. The bone reamingswere saved and used as graft materials. The reamed channel was thentapped followed by insertion of a 10 millimeter-diameter titanium BAKdevice (SpineTech, Minneapolis, Minn.). No attempt was made to excisethe posterior longitudinal ligament or expose the spinal canal.

The goats were divided into three treatment groups consisting of sevengoats each. Group I had a device filled with autogenous bone graftharvested from the reamings at each disc level. Group II utilized ahydroxyapatite-coated implant filled with autogenous bone reamings asgraft. Group III utilized a device filled with a collagen spongeimpregnated with 200 μg of recombinant BMP-2 (Genetics Institute,Cambridge, Mass.). Prior to installation of the devices, wounds wereirrigated with a solution of normal saline, bacitracin (50U/cc),polymyxin B (0.05 mg/cc), and neomycin (0.5%). The longus coli musclewas then closed with a running suture. The subcutaneous tissue wasreapproximated with interrupted sutures and the skin with a runningsuture.

Post-operatively the animals were maintained under observation untilfully recovered from general anesthesia. They received two doses ofNaxcell (ceftiofur), 500 mg intravenously propetatively and 500 mgintramuscularly post-operatively. A soft bandage was applied to theanimals neck, and they were allowed ad lib activity under dailyobservation in a pen for several days.

Clinical evaluation was performed every three weeks. Lateral cervicalspine radiographs were obtained immediately post-operatively and atthree, six and nine weeks. Fluorochrome labels were administered atthree, six and nine weeks. These consisted of oxytetracycline (30 mg/kgIV) at three weeks, alizarin complex one (30 mg/kg IV) at six weeks, andDCAF (20 mg/kg IV) at nine weeks. At twelve weeks, the goats wereeuthanized by an intravenous injection of Beuthanasia. The cervicalspine was then excised, and all surrounding tissues removed from it. Thespecimen was then radiographed in the AP and lateral planes.

Biomechanical Testing: The spine specimens were brought fresh to thebiomechanics laboratory for biomechanical testing. The spines weremounted into frames at C2 and C7 with a polyester resin (Lite Weight 3Fiberglass-Evercoat, Cincinnati, Ohio). The biomechanical tests wereperformed on a modified MTS Bionix 858 Servo-Hydraluic Material Tester(MTS Corporation, Minneapolis, Minn.). The MTS machine can apply axialcompressive and torsional loads about the longitudinal axis of thespine. This system allows a constant bending moment to be applieduniformly over the length of the spine resulting in a pure sagittalflexion and extension load, with axial load and torsion maintained atzero.

Separate tests were performed for axial compression, torsion,flexion-extension, and lateral bending. Axial load was cycled from 0 to100 N in compression. Coupled motion in rotation or sagittal bending wasallowed. Torsion was cycled from positive to negative 5 N-m with a 50 Ncompressive preload. Again, coupled motion was allowed by leaving axialload and sagittal bending in load control. Sagittal bending was cycledfrom flexion to extension with a uniform 2 N-m bending moment with a 5 Ntensile preload. Lateral bending was performed from left to right with auniform 2 N-m bending moment with a 5 N tensile preload. Each testconsisted of five sinusoidal load cycles at 0.1 Hz. Specimens werepreconditioned over the first four cycles with data from the fifth cycleused for analysis. Data acquisition was continuous throughout each testand stored in a computer data file.

Axial compressive data included axial load (N) and axial displacement(mm). Flexion-extension, torsional, and lateral bending data includedaxial load (N), torque (N-m), and rotational displacement (degrees). Themeasurement of axial, flexion-extension, lateral bending and torsionaldisplacement was performed simultaneously using extensometers appliedacross each of the operated disc levels. Data analysis consisted ofstiffness calculation across each disc space for axial load,flexion-extension, torsion, and lateral bending.

Radiographic Analysis: Analysis was carried out on all of the three,six, nine and twelve week radiographic films. The radiographs wereanalyzed for cage migration and the absence or presence of lucent linessurrounding each cage. If a lucent line was seen on either the AP orlateral radiograph, that cage was noted to possess a lucency.

Histologic Analysis: Following biomechanical testing specimens wereremoved from the mounting grips and frames. The spines were cut throughthe mid-axial portion of the C3-, C4, and C6 vertebral bodies thusproviding three individual specimens containing the implant in abone-disc space-bone block. The specimens were then cut into sagittalsections starting on the right lateral side using an Isomet Plusprecision saw (Buehler Instruments, Lake Bluff, Ill.). When the sagittalslice revealed the first sign of the cage, two additional 2.5 mm sliceswere removed. These two slices were then stores in 70 percent alcoholawaiting microradiographic analysis. A third sagittal slice was thenremoved and set aside for fluorochrome analysis. The remaining specimenis stored in 70 percent alcohol.

The first two slices that contain the cage were then processed formicroradiographs. A sagittal microradiograph was obtained in a HewlettPackard Faxitron unit (Hewlett Packard, McMinnville, Oreg.). Eachsagittal microradiograph contained two cage-vertebral body interfaces.Each of these interfaces was graded separately and as to whether or notthere was bone or fibrous tissue surrounding the cage. Each interfacewas then subclassified as to whether or not there was bone growth intothe cage from the respective interface. Thus each discinterspace-cage-end plate junction could be classified as either: (1)cage completely surrounded by bone with bone ingrowth (B-B), (2) cagecompletely surrounded by bone with fibrous or no ingrowth (B-F/E), (3)cage surrounded by fibrous tissue with fibrous ingrowth (F-F), or (4)cage surrounded by fibrous tissue and empty (F-E).

The presence or absence of a successful arthrodesis was determined fromthe sagittal microradiographs. If both disc interspace-cage-end plateinterfaces were completely surrounded by bone and there was boneconsolidation throughout the interspace, then the level was deemed tohave a solid arthrodesis. If both interfaces were surrounded by fibroustissue and the cage was empty, then level was deemed to have a failedarthrodesis. If one interface was surrounded by bone and the other withfibrous tissue, or if both interfaces were surrounded by fibrous tissueand the cage filled with fibrous tissue, then the level was deemed tohave an intermediate result.

The third sagittal slice was mounted in polymethylmethacrylate forfluorochrome analysis. Using the Isomet Plus saw, 200 to 360 μm thickslices were obtained. These slices were then ground to a thickness of100 μm using a Maruto ML-512D Speed Lapping machine (Maruto Instruments,Tokyo, Japan). A sagittal microradiograph was obtained of the specimenat a thickness of 100 μm to correlate with the fluorochrome analysis.After obtaining this microradiograph the slice was ground down to athickness of 40 μm and mounted on a slide for fluorochrome analysis. Thepresence or absence of each marker around and within the cage allowed usto determine the relative time frame of bone revascularization aroundand within the cage.

RESULTS: All 21 goats successfully underwent surgery and survivedwithout difficulty during the length of the experiment. No cervicalspine wound infection occurred. There were no neurologic complications.

Radiographic Results: None of the cages in any of the groups displaced.In group I there were three cages with lucencies. In group II there werefour cages with lucencies. In group II none of the 21 cages exhibitedany lucencies.

Microradiograph Results: The results of grading each individualcage-endplate-interface junction are summarized in Table I. The BMPfilled cages had a greater number of interfaces surrounded by bone and agreater amount with bone ingrowth than either of the other two groups.

The arthrodesis success rate was greatest for the BMP filled cages at95% followed by the HA coated (62%) and standard devices (48%). Thisdifference was statistically significant (p=0.002). The unsuccessfularthrodesis rate was 14% for both HA coated and standard groups, andzero for the BMP filled cages. The intermediate results were 38% for thestandard cage, 14% for the hydroxyapatite cage, and 5% for the BMPfilled cage.

Biomechanical Data: Mean biomechanical stiffness data in axialcompression, torsion, flexion, extension, and lateral bending issummarized by group in Table II. There were no statistical differencesby group in any of the loading modes tested. While there were nostatistically significant differences in stiffness in any loading modeby arthrodesis result, there was a tendency for a cage with a successfularthrodesis to be stiffer than a failed arthrodesis in axialcompression, torsion, flexion, and extension.

Fluorochrome Analysis: There were ten cages in group I that exhibitedbone formation completely around the cage. Seven of these cages (70%)exhibited bone revascularization after the three week injection andthree (30%) after the six week injection. In group II, thirteen cagesexhibited bone formation completely around the cage. Either of these(62%) exhibited revascularization after the three week injection, three(23%) after the six week injection, and two (15%) after the nine weekinjection. In group III, twenty cages exhibited bone formationcompletely around the cage. Nineteen of these (95%) exhibited bonerevascularization after the three week injection and one (5%) after thesix week injection.

Twenty-two of the sixty-three cages in all three groups exhibited bonegrowth within the cage. In group I, one cage of six (17%) exhibited bonerevascularization after the six week injection, and five cages (83%)after the nine week injection. In group II all five cages exhibited bonerevascularization after the nine week injection. In group III, three ofeleven ages (27%) exhibited bone revascularization after the three weekinjection, six (55%) after the six week injection, and two (18%) afterthe nine week injection. Thus, in general, the BMP filled cagesexhibited earlier revascularization of bone both around and within thecages compared to the other two groups.

CONCLUSION: The use of an intervertebral fusion cage filled with BMPresulted in a much higher arthrodesis rate and accelerated bonerevascularization compared to either autogenous bone filled devices, orautogenous interbody bone grafts with or without plate stabilization.

EXAMPLE 2

Design: Twelve mature female sheep underwent single level midlumbarinterbody fusion. All surgical dissections were performed in anidentical fashion. Following preparation of the anterior fusion sitesthe implants were inserted. Sheep were treated with a Threaded InterbodyFusion Device (TIBFD) containing rhBMP-2 carried on a type I fibrillarcollagen (Helistat)(n=6) in a single cage, lateral orientation through aretroperitoneal approach. Previous limbs of the study (all n=6) includedTIBFD with autogenous bone plugs, autogenous bone plugs alone, or sham(empty) fusion sites. The sheep were allowed to graze immediatelypost-operatively and no external immobilization was used. All animalswere sacrificed six months following surgery. Fourteen additionalcadaver sheep spines had been obtained to determine baselineintervertebral mechanical stiffness measures.

Materials: The interbody fusion cages developed and manufactured bySofamor Danek, Inc., Memphis Tenn. were made of Ti-6Al-4V alloy anddesigned as closed cylinders. The devices were 14 mm in diameter andcontained a screw-in endcap to allow for placement of graft materials.The device porosity as described by the manufacturer was 35% overallhole to metal ratio with increased porosity in contact with theintervertebral bodies. The mechanical load to yield is reported to be80.000 Newtons (maximum human physiologic loads—10.000 Newtons). Cycliccompressive loading from 800 to 9.680 Newtons at 15 Hz over 5.000.000cycles resulted in no observable microscopic damage or deformation.

The dose of rhBMP-2 was 0.43 mg/ml. The protein in its buffered solutionwas drip applied to commercial grade type I collagen (Helistat). Thecomposite was then inserted into the cage chamber following which thecage cap was applied. The device was then inserted into the preparedfusion site.

Surgical procedure: A 10 cm rostral to caudal left flank incision wasmade under sterile conditions. Following incision of the lateral fasciaof the external abdominal musculature, the retroperitoneal plane wasidentified. Proceeding through this plane the intervertebral discbetween the L4 and L5 veterbral bodies was cleaned of soft tissue.Segmental vessels were not ligated unless required for additionalexposure. The descending aorta was retracted to expose the anteriorlongitudinal ligament and anterior annulus. A 2 mm guide wire was placedtransversely through the intervertebral disc bisecting the disc in thesagittal plane. A cannulated trephine punch was then used over the wireto create a left lateral annulotomy.

A blunt tip “bullet” shaped dilator 7 mm in diameter was used over thesame wire to expand the disc space and place the annulus under tension.A four-prong outer sleeve was placed over the distractor and impacted soas to purchase the adjacent vertebral bodies. Side prongs in the discspace aided in maintaining distraction. The dilator was then removed. Abone cutting reamer was placed through the outer sleeve and used tocreate a transverse hole through the disc space. At least 3 mm ofendplate and subchondral bone of the adjacent vertebral bodies wereremoved during the process. At this point the device was prepared andimplanted. Routine closure of external abdominal muscular fascia,subcutaneous tissue and skin was performed.

Mechanical Testing: All sheep that had undergone surgery weremechanically tested for fusion stiffness following sacrifice. Inaddition, cadaver spines from fourteen untreated sheep were also testedto establish baseline parameters for the L4-L5 motion segment. The L4-L5intervertebral segments (fusion sites) were tested for stiffness tosagittal and coronal plane bending moments (flexion, extension, rightbending, left bending) in all eighteen sheep. For baseline measures,fourteen untreated cadaver sheep spines were also tested for stiffnessat the L4-L5 intersegment in the same planes of motion.

Following sacrifice, the spinal columns from L3 to L6 were explanted.Intersegmental ligamentous tissues were retained. The transverseprocesses were trimmed to facilitate polymethylmethacrylate (PMMA)potting of the L3 and L6 vertebrae. The PMMA pots did not include theL3-L4 or the L5-L6 discs.

Non-destructive mechanical tests were performed with an MTS 812servohydraulic testing machine. The specimen was mounted in an apparatussuch that it was oriented perpendicular to the axis of actuation. Oneend of the specimen was fixed while the other was free to move andplaced directly above the actuator. Pure bending moments were appliedusing a system of cables and pulleys. Rotational variable differentialtransformers (RVDT) were attached to the vertebral body via bone screwsto measure rotation in the L4-L5 motion segment and to the free end tomeasure its angle with respect to horizontal load-displacement data wererecorded.

For each test, loads were applied in three cycles consisting of a 5second ramp per cycle with a maximum applied moment of approximately 10N-m. Tests were performed in flexion, extension, right bending, and leftbending modes sequentially. Stiffness was calculated as the slope of theforce versus angular displacement curve at 8 N-m for all groups.

Radiographic Evaluation: Under general anesthesia, anterior-posteriorand lateral radiographs were obtained immediately post-operatively, andthen two months, four months, and six months following surgery.Measurements of vertebral body heights and disc heights along the lumbarspine were made in the mid-sagittal line using a photo image analyzer(superfine pitch monitor, Image-1/Atsoftware. 1991). All measurementswere made on true lateral radiographs. Since measures of the interbodydisc heights at the fusion sites were obscured by implant materials and“interbody height index” (IB index) was calculated to reflect interbodydistraction. This index was calculated as follows: The mid-sagittal spanof the fused segments from the cephalad endplate of L4 to the caudalendplate of L5 was measured as the “fusion height”. Since the vertebraewere of relatively uniform height, the sum of the mid-sagittal heightsof the L3 and L6 vertebrae was used to estimate the some of the heightsof the L4 and L5 vertebrae excluding the intervening intervertebraldisc. The sum of the L3 and L6 vertebrae was then subtracted from thefusion height to ascertain the “calculated interbody height”. In orderto correct for differences in magnification this value was expressed asa ratio to average vertebral height and this value was defined as the IBindex.

Results: The mechanical testing results from one specimen implanted withTIBFD+rhBMP-2 were excluded due to apparatus errors.

Results of Mechanical Testing Data: Means, standard deviations as afunction of group are presented in Table III. Results from overall andpairwise statistical comparisons are presented in Table IV. Meanstiffness was significantly different among the groups (two treatmentand unoperated control) for each mode of testing (P=0.005, P=0.0001,P=0.0001, P=0.0001).

All surgically treated intersegments were significantly stiffer thanuntreated intersegments. That is, sites implanted with TIBFD+rhBMP-2 orTIBFD+autograft compared to those untreated were significantly stifferto flexion (P=0.0001, P=0.055) extension (P=0.0001, P=0.0001) rightbending P=0.0001, P=0.0001) and left bending moments (P=0.0001,P=0.0001). There was no difference in stiffness between intersegmentstreated with TIBFD+rhBMP-2 and those treated with TIBFD+autograft(comparisons for all modes of testing were P 0.05).

Results of Interbody Height Measures Interbody Height Index: Meansstandard deviations and results from overall and pairwise statisticalcomparisons are presented in Table V. There is no differences in theInterbody Height index between TIBFD+rhBMP-2 and TIBFD+autograft at eachof the time measures F(4.40)=0.20 P=94). Subsidence occurred primarilyin the first two post-operative months in both groups (roughly 20% ofthe initial interbody disc height) although the decrease in interbodyheight was not significant (F(2.20)=0.19, P=0.83).

Conclusions: No differences were noted either mechanically ormorphologically between the fusions created with TIBFD+rhBMP-2 and thosecreated with TIBFD+autograft. There was a trend toward greater stiffnessto flexion with TIBFD+rhBMP-2 but this was not significant. Subsidencetended to occur in both groups in the first two months. Harvesting ofautogenous bone graft provides no advantage compared to the use ofrhBMP-2 with type I fibrillar collagen in this model.

EXAMPLE 3

Open Porosity Polylactic Acid Polymer (OPLA) is provided in sterilepackaged 12.0 mm×6.5 mm×30 mm strips (two strips per package). The pureOPLA is sterilized via gamma irradiation. The rhBMP-2 is provided infreeze-dried powder form and reconstituted intra-operatively in sterilewater and supplemented with a buffer vehicle solution. The rhBMP-2 isintroduced into the carrier material and the carrier is placed into thehollow interior of a metal fusion cage device. The device is thenimplanted at the fusion site.

EXAMPLE 4

A rhBMP-2/collagen implant is prepared from Helistat® AbsorbablyCollagen Hemostatic Agent (Integra LifeSciences Corporation) andrhBMP-2. The collagen carrier is placed within the hollow interior of ametal fusion cage device. The device is implanted at the fusion site.TABLE I Individual Cage-Interspace-Endplate Bone Ingrowth Results byCage Group Microradiograph Grade* Group B-B B-F/E F-F F-E I 33% 29% 14%24% II 26% 43% 12% 19% III 53% 45%  0%  2%*See text for definition of each grading result.

TABLE II Biomechanical Stiffness Data by Cage Group Axial CompressionTorsion Flexion Extension Lateral Bending Group (N/mm) (N-m/degree)(N-m/degree) (N-m/degree) (N-m/degree) I 187 (92)   8.4 (11.7) 0.99(0.91) 5.0 (7.2) 1.4 (2.2)  II 165 (70)  10.2 (12.5) 1.6 (2.7) 3.4 (2.8)2.3 (3.9)  III 313 (388)  6.7 (10.2) 0.96 (0.48) 3.1 (2.4) 1.0 (0.66) pvalue 0.46 0.32 0.24 0.82 0.72Values in parenthesis represent standard deviations

TABLE III Results of Mechanical Testing Flexion Extension Rt. BendingLt. Bending Conditions n Mean ± sd Mean ± sd Mean ± sd Mean ± sd TIBFD +rhBMP-2  5* 15.91 ± 6.90  25.19 ± 10.91 19.35 ± 5.82  15.40 ± 2.35 TIBFD + autograft  6 11.00 ± 7.81  24.55 ± 10.51 9.89 ± 4.04 19.47 ±8.56  Untreated 14 6.71 ± 1.40 6.03 ± 2.15 0.41 ± 0.11 4.04 ± 0.90 25

TABLE IV Results of Mechanical Testing Flexion Extension Right BendingLeft Bending Mean ± sd. P Mean ± sd. P Mean ± sd. P Mean ± sd. PCompared Conditions TIBFD + rhBMP-2 15.91 ± 6.90 (P = 0.30) 25.19 ±10.91 (P = 0.92) 19.35 ± 5.82 (P = 0.36) 15.40 ± 2.35 (P = 0.33) TIBFD +autograft 11.00 ± 7.81 24.55 ± 10.51 15.58 ± 9.89 19.47 ± 8.56 TIBFD +rhBMP-2 15.91 ± 6.90 (P = 0.0001) 25.19 ± 10.91 (P < 0.0001) 19.45 ±5.82 (P < 0.0001) 15.40 ± 2.35 (P < 0.0001) Untreated  6.71 ± 1.40 6.03± 2.15  2.98 ± 0.41  4.04 ± 0.90 TIBFD + autograft 11.00 ± 7.81 (P =0.06) 24.55 ± 10.51 (P < 0.0001) 15.58 ± 9.89 (P < 0.0001) 19.47 ± 8.56(P < 0.0001) Untreated  6.71 ± 1.40 6.03 ± 2.15  2.98 ± 0.41  4.04 ±0.90

TABLE V Results Interbody Height Index: from 0 to 6 months post op 2months 4 months 6 months Conditions n Mean ± sd Mean ± sd Mean ± sd Mean± sd TIBFD +  6* 0.20 ± 0.14 ± 0.03 0.17 ± 0.04 0.15 ± 0.03 rhBMP-2 0.04TIBFD +  6 0.20 ± 0.15 ± 0.05 0.15 ± .05  0.16 ± .05  autograft 0.03Total 12 measured

EXAMPLE 5

Testing Rationale

Testing was conducted on endcaps to measure the resistance of the endcapto expulsion by a rhBMP-2 soaked collagen sponge and to compare theresistance to a known polyethylene endcap.

Test A

Press-Fit Endcap Pushout Test

This test was conducted to determine the static force required todislodge a polyethylene press-fit endcap from a BAK™ (Spine Tech,Minneapolis, Minn.) device. The endcap was snap-fit to the BAK™ deviceand an axial load was applied through the cavity of the BAK™ device tothe endcap. The push-out load for five (5) samples ranged from 12 to 37pounds of force.

Test B

Test Set-Up and Methods

Five (5) samples of a titanium 12 mm endcap (894-120, Sofamor Danek,USA) (894-XXX, Sofamor Danek, USA, Memphis, Tenn.) were each placed intoa 12 mm titanium NOVUS™LT (Sofamor Danek, USA) implant as shown in FIGS.18 and 19. The 12 mm implant was fixed rigidly to the table of a closedloop servohydraulic test machine. The actuator of the testing machinewas attached to the endcap via an adaptor which was threaded into theendcap. An axial load was applied to pull the endcap out at a rate of 25mm/min until the endcap was completely removed from the 12 mm implant.The data, including maximum load and displacement, were recorded andplotted using Superscope II data acquisition software.

Results

All endcaps pulled out via elastic deflection of the two anchor prongs.The mean pull-out load was 187N (41.99 lbf). Table 1 shows the raw datafor the pull-out tests.

Test C

The methods of Test B were repeated on nine (9) samples except that theload was applied at a rate of 12.5 mm/min. The mean pull-out load was30.57 Mean Force in Pounds. The 30.57 value compares well to the Test Bvalue of 41.99 The sample size for this testing was 9, while the samplesize of Test B was 5.

DISCUSSION AND CONCLUSIONS

The testing results show that the endcap of this invention is resistantto explusion in vivo for two reasons. First, it is well known that theintervertebral disc is under complex, combined loading. However, none ofthe loads acting on the disc space would act directly on the endcap ofthe implant in order to cause endcap explusion. Secondly, it is unlikelythat the rhBMP-2 soaked collagen sponge could exert 177 N (41.99 lbf) offorce to expulse the endcap.

The anchor prong endcaps of this invention were easily inserted into thedevices by hand. In one instance, the endcap was inserted via theservohydraulic test machine. The insertion load was measured and foundto be 3.2 lbf. This provides additional support for the solid endcapengagement. The average expulsion force is 13 times the insertion load.

The anchor prong endcaps of this invention compared very favorably to aknown polyethylene press-fit endcap design. The press-fit cap averaged25 pounds force with a range of 12 to 37 pounds. The anchor prong cap ofthis invention exceeded those values with a mean of 30.57 pounds and arange of 12.5 to 46.62 pounds of force over nine (9) samples.

While the invention has been described in detail in the foregoingdescription, the same is to be considered as illustrative and notrestrictive in character, it being understood that only the preferredembodiments have been shown and described, and that all changes andmodifications that come within the spirit of the invention are desiredto be protected.

1. A cap for blocking the opening of a hollow fusion device defining athru-hole, comprising: an occlusion body sized and shaped for blockingthe opening; and an elongate anchor projecting from said occlusion body,said anchor including a first end attached to said occlusion body and anopposite second end having a lip for engaging the thru-hole, said anchorhaving a length which reaches from said occlusion body to the thru-holewhen the cap is inserted into the opening and said lip is engaged tosaid thru-hole.
 2. The cap of claim 1 further comprising a flangeprojecting from a perimeter of said occlusion body.
 3. A fusion devicefor facilitating arthrodesis in the disc space between adjacentvertebrae, comprising: a hollow load bearing body having an internalsurface defining a chamber for an osteogenic material and an opening incommunication with said chamber, said load bearing body having a lengthand a first diameter at a first end sized to be greater than the spacebetween the adjacent vertebrae, said load bearing body having an outersurface with a pair of opposite cylindrical portions and a pair ofsubstantially flat opposite side walls between said opposite cylindricalportions, said side walls extending along a substantial portion of saidlength of said load bearing body; and a cap for blocking said opening,said cap having an occlusion body sized and shaped for contacting saidopening, and an elongate anchor projecting from said occlusion body,said anchor including a first end attached to said occlusion body and anopposite second end having engaging means for engaging said load bearingbody to hold said occlusion body within said opening.
 4. The device ofclaim 3 wherein said outer surface defines a thru-hole in communicationwith said chamber and said anchor has a length which reaches from saidocclusion body to said thru-hole when said cap is inserted into saidopening and said lip is engaged within said thru-hole.
 5. The device ofclaim 3 further comprising a flange projecting from a perimeter of saidocclusion body.
 6. A fusion device for facilitating arthrodesis in thedisc space between adjacent vertebrae, comprising: a hollow load bearingbody having an internal surface defining a chamber for an osteogenicmaterial and an opening in communication with said chamber, said loadbearing body having a length and a first diameter at a first end and alarger second diameter at a second end opposite said first end, saidfirst and second diameters sized to be greater than the space betweenthe adjacent vertebrae, said load bearing body having an outer surfacetapered from said first diameter to said second diameter; and a cap forblocking said opening, said cap having an occlusion body sized andshaped for contacting said opening, and an elongate anchor projectingfrom said occlusion body, said anchor including a first end attached tosaid occlusion body and an opposite second end having engaging means forengaging said load bearing body to hold said occlusion body within saidopening.
 7. The device of claim 6 wherein said outer surface defines athru-hole in communication with said chamber and said anchor has alength which reaches from said occlusion body to said thru-hole whensaid cap is inserted into said opening and said lip is engaged withinsaid thru-hole.
 8. A tool for manipulating a cap for a hollow interbodyfusion device, comprising: a pair of prongs each having a proximal enddefining first engaging means for engaging the fusion device; a shafthaving a first end defining second engaging means for engaging the cap;and means for slidably supporting said shaft between said prongs.
 9. Atool for manipulating a cap for a hollow interbody fusion device,comprising: a body defining a passageway therethrough; a pair of prongseach having a distal end attached to said housing and a proximal endhaving facing engagement surfaces for engaging the fusion device; and ashaft slidably disposed within said body, said shaft having a first enddefining a cap-engaging tip for engaging a tool hole in the cap, saidshaft slidable between a retracted position and an extended position atwhich said first end is adjacent and between said proximal ends of saidprongs.
 10. The tool of claim 9 wherein said prongs are made of aresilient material.
 11. The tool of claim 9 wherein said cap engagingtip defines threads.
 12. The tool of claim 9 further comprising: a pairof releasing members, one of said releasing members disposed on each ofsaid facing engagement surfaces, said releasing members having a heightand a width for being insertable into apertures in a body wall in thefusion device.
 13. The tool of claim 9 further comprising releasingmeans for applying pressure to elongate arms of the cap to deflect thearms inwardly to release the cap from the interbody fusion device. 14.The tool of claim 13 wherein said releasing means includes a pair ofreleasing members, one of said releasing members disposed on each ofsaid facing engagement surfaces, said releasing members having a heightand a width for being insertable into apertures in a body wall in thefusion device to disengage the elongate arms from the apertures.
 15. Thetool of claim 9 further comprising: a proximal stop member disposed onsaid shaft adjacent said first end for preventing said first end fromentering said passageway.
 16. The tool of claim 15 wherein said proximalstop member comprises an O-ring engaged to said shaft.
 17. The tool ofclaim 16 wherein said shaft defines a groove for seating said O-ring.18. The tool of claim 9 further comprising a distal stop member attachedto a second end of said shaft, said distal stop member having aperimeter which is larger than a perimeter of said passageway to preventsaid second end of said shaft from entering said passageway.
 19. Thetool of claim 17 further comprising a distal stop member attached to asecond end of said shaft, said distal stop member having a diameterwhich is larger than a diameter of said passageway to prevent saidsecond end of said shaft from entering said passageway.
 20. The tool ofclaim 9 further comprising a distal shaft manipulating member attachedto a second end of said shaft for rotating and sliding said shaft withinsaid passageway.
 21. The tool of claim 20 wherein said manipulatingmember has a dimension that is larger than a perimeter of saidpassageway to prevent said second end of said shaft from entering saidpassageway.
 22. The tool of claim 21 wherein said manipulating member isa thumb wheel.
 23. A fusion device for facilitating arthrodesis in thedisc space between adjacent vertebrae, comprising: a metal body composedof a porous, biocompatible bone ingrowth material having interconnectedcontinuous pores; and an osteogenic material within the pores of saidbone ingrowth material, said osteogenic material including a bonemorphogenetic protein.
 24. The device of claim 23 wherein said bonemorphogenetic protein is a recombinant human protein.
 25. The device ofclaim 24 wherein said bone morphogenetic protein is rhBMP-2, rhBMP-7 ora mixture thereof.
 26. The device of claim 23, further comprising abonding layer provided over said metal body, said layer extending intosaid pores.
 27. The device of claim 26 wherein said bonding layerincludes a calcium phosphate composition.
 28. The device of claim 27wherein said calcium phosphate composition comprises hydroxyapatite. 29.The device of claim 28 wherein said calcium phosphate compositionfurther comprises tricalcium phosphate, said calcium phosphatecomposition being a biphasic ceramic.
 30. The fusion device according toclaim 23 wherein said porous material is a composite comprising anonmetallic rigid foam substrate formed by an interconnected network ofcarbonaceous material defining continuous, interconnected pores and ametallic film substantially covering said interconnected network. 31.The fusion device according to claim 30 wherein said carbonaceousmaterial is a carbon foam.
 32. The fusion device according to claim 30wherein said metallic film comprises a group VB metal or an alloy ofsaid group VB metal.
 33. The fusion device according to claim 32 whereinsaid metallic film comprises tantalum or an alloy thereof.
 34. Thedevice of claim 30 further comprising a bonding layer of a calciumphosphate composition, said calcium phosphate composition layered oversaid interconnected network.
 35. The device of claim 23 wherein saidbody: is elongated having a length, a first diameter at a first end anda larger second diameter at a second end opposite said first end, saidfirst and second diameters sized to be greater than the space betweenthe adjacent vertebrae; said body having an outer surface that issubstantially continuously tapered from said first end to said secondend with external threads defined on said outer surface and extendingsubstantially entirely along said length of said body.
 36. The device ofclaim 23 wherein said body: is elongated having a length between a firstend and a second end thereof, and a first diameter at said first endsized to be greater than the space between the adjacent vertebrae, saidbody having an outer surface with a pair of opposite cylindricalportions extending along substantially the entire length of said bodyand defining said first diameter, and a pair of substantially flatopposite side walls connected between said opposite cylindricalportions, said side walls extending along a substantial portion of saidlength of said body.
 37. The device of claim 36 further comprisingexternal threads defined on said pair of opposite cylindrical portionsof said outer surface and extending along substantially the entirelength of said body.
 38. The device of claim 37 wherein said cylindricalportions are tapered along a substantial portion of said length anddefine a second diameter at a second end thereof that is greater thansaid first diameter.
 39. A fusion device for facilitating arthrodesis inthe disc space between adjacent vertebrae, comprising: a hollow loadbearing body having an internal surface defining a chamber and anopening in communication with said chamber; and an osteogenic materialplaced within said chamber, said osteogenic material including a bonemorphogenetic protein in a suitable carrier.
 40. The device of claim 39wherein said bone morphogenetic protein is selected from the groupconsisting of BMP-1, BMP-2, BMP-3, BMP-4, BMP-5, BMP-6, BMP-7, BMP-8,BMP-9, BMP-10, BMP-11, BMP-12, BMP-13.
 41. The device of claim 40wherein said carrier is selected from the group consisting of calciumsulphate, polylactic acids, polyanhydrides, collagen, calcium phosphatesand polymeric acrylic esters.
 42. The device of claim 41 wherein saidcarrier is a sponge.
 43. The device of claim 39 wherein said carrier isa biphasic ceramic including hydroxyapatite and tricalcium phosphate.44. The device of claim 39 wherein said load bearing body comprises acalcium phosphate composition.
 45. The device of claim 44 wherein saidload bearing body comprises hydroxyapatite.
 46. The device of claim 45wherein said load bearing body comprises biphasichydroxypatite/tricalcium phosphate.
 47. The device of claim 39 furthercomprising a cap for blocking said opening, said cap including: anocclusion body sized and shaped for contacting said opening andincluding, an outer wall defining osteogenic apertures, said aperturessized to permit bone ingrowth and protein egress, an opposite innersurface; and a rim in communication with said outer wall and said innersurface, said rim defining an engaging surface for contacting saidopening; and an elongate anchor projecting from said rim, said anchorincluding a first end attached to said rim and an opposite second endhaving a lip, said lip projecting from said second end to contact saidinternal surface to hold said occlusion body within said opening. 48.The device of claim 47 further comprising a flange projecting from aperimeter of said occlusion body.
 49. The device of claim 39 whereinsaid load bearing body: is elongated having a length between a first endand a second end thereof, and a first diameter at said first end sizedto be greater than the space between the adjacent vertebrae, said loadbearing body having an outer surface with a pair of opposite cylindricalportions extending along substantially the entire length of said loadbearing body and defining said first diameter, and a pair ofsubstantially flat opposite side walls connected between said oppositecylindrical portions, said side walls extending along a substantialportion of said length of said load bearing body.
 50. The device ofclaim 48 further comprising external threads defined on said pair ofopposite cylindrical portions of said outer surface and extending alongsubstantially the entire length of said load bearing body.
 51. Thedevice of claim 49 further comprising a cap for blocking said opening,said cap having: an occlusion body sized and shaped for contacting saidopening; and an elongate anchor projecting from said occlusion body,said anchor including a first end attached to said occlusion body and anopposite second end having engaging means for engaging said load bearingbody to hold said occlusion body within said opening.
 52. The device ofclaim 51 wherein said occlusion body includes an outer wall definingosteogenic apertures sized to permit bone ingrowth and protein egress.53. The device of claim 51 wherein said engaging means comprises a lipprojecting from said second end to contact said internal surface. 54.The device of claim 51 wherein said load bearing body defines athru-hole in communication with said chamber and said anchor has alength which reaches from said inner surface to said thru-hole when saidcap is inserted into said opening and said lip is engaged within saidthru-hole.
 55. The device of claim 53 wherein said outer wall is flushwith said opening when said lip is engaged to the thru-hole.
 56. Thedevice of claim 51 further comprising a second elongate anchorprojecting from said occlusion body.
 57. The device of claim 51 whereinsaid anchor is composed of a resilient material.
 58. The device of claim51 wherein said cap is composed of a biocompatible polymer.
 59. Thedevice of claim 58 wherein said polymer is polyethylene.
 60. The deviceof claim 56 wherein said occlusion body defines a tool hole forreceiving a driving tool.